Climate change FAQs

Through a question and answer approach, the Climate Change FAQs will explain some basic knowledge and facts of climate change in layman terms in order to enhance the public's understanding of the causes of climate change, its impacts and what we can do to mitigate its effects.

Although both "Weather" and "Climate" are used to describe the condition of the atmosphere, they are very different in terms of the time scale considered. "Weather" describes the combined atmospheric situation in a place at the time or within a very short time (several hours to a few days), such as wind speed, temperature, cloud amount, rainfall, pressure, etc. "Climate" refers to the average of the meteorological condition and pattern in a place over a longer period of time. In other words, "Climate" can be described as the "Average Weather". According to the definition of the World Meteorological Organization (WMO), the reference period for compiling the climate statistics should be at least 30 years.

According to the Intergovernmental Panel on Climate Change (IPCC), climate change refers to any change in climate over time, whether due to natural variability or as a result of human activity. This usage differs from that in the United Nations Framework Convention on Climate Change (UNFCCC), where climate change refers to a change of climate that is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and that is in addition to natural climate variability observed over comparable time periods.

The Intergovernmental Panel on Climate Change (IPCC), under the auspices of the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), is a scientific body tasked to evaluate the risk of climate change caused by human activity. Climate change is a very complex issue. Policymakers need an objective source of information about the causes of climate change, its potential environmental and socio-economic consequences, and the adaptation and mitigation options to respond to its impact. This is the key motivation behind the establishment of IPCC in 1988 as the authority on climate change.

The main activity of IPCC is in the compilation of assessment reports on a regular basis. The First Assessment Report in 1990 played a decisive role in the establishment of the United Nations Framework Convention on Climate Change (UNFCCC). The Second Assessment Report in 1995 provided key input for the negotiations of the Kyoto Protocol. The Third Assessment Report in 2001 and a number of special reports provided relevant information for the development of the UNFCCC and the Kyoto Protocol. The Fourth Assessment Report in 2007 confirmed that warming of the climate system was unequivocal. The Fifth Assessment Report in 2013 reaffirmed this finding and concluded that it was extremely likely human influence had been the dominant cause behind the observed warming since the mid-20th century.

The heat content at the surface of the earth is mainly derived from the sun. When solar radiation mainly in the form of visible light reached the earth, it heats up the earth. To balance the absorbed incoming energy, the earth will radiate the same amount of energy of infra-red radiation back to space. Greenhouse gases in the atmosphere, such as carbon dioxide, methane and nitrous oxide, absorb part of the infra-red radiation emitted from the earth and then return part of the re-emitted radiation to the earth. This is so-called the greenhouse effect. As such, greenhouse gases act like a blanket and prevent the earth from losing excessive heat. If the greenhouse gases concentration in the atmosphere increases, the earth surface will retain more heat than before, the air temperature of the earth surface will rise. But, if greenhouse gases are not present in the earth, its average surface temperature would be very low of around -18oC rather than the about 14.5oC found today.

Rapid development of economic and industrial activities since the 18th century has lead to excessive use of energy and resources. In particular, the burning of fossil fuels (such as coal and oil) emits large amounts of greenhouse gases into the atmosphere. The increase in anthropogenic (human induced) greenhouse gas concentrations in the atmosphere enhances the greenhouse effect. This can be visualized as the thickening of an invisible blanket covering the earth, resulting in global warming. The human impact on climate during this era greatly exceeds that due to known changes in natural processes, such as solar changes and volcanic eruptions.

The main greenhouse gases added through human activities, including carbon dioxide, methane and nitrous oxide, will reside in the atmosphere for decades or even centuries. The resulting global warming and its effect are thus long lasting. As such, it is considered by scientists and policy makers to be one of the most serious problems that mankind has to face, not only now but for generations to come.

According to the Fourth Assessment Report of IPCC, the global mean temperature rose by 0.74 during the hundred year period between 1906 and 2005. The rising rate was 0.13 per decade in the 50 years from 1956 to 2005, nearly twice the rate in the past 100 years. Eleven (1998, 2005, 2003, 2002, 2004, 2006, 2007, 2001, 1997, 2008, 1999) of the last twelve years (from 1997 to 2008) rank among the 12 warmest years on record.

Observed changes in global average surface temperature. Changes are relative to corresponding averages for the period 1961-1990. Smoothed curve represents decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals. (Source: IPCC, 2007)

Precipitation is the general term for rainfall, snowfall and other forms of frozen or liquid water falling from clouds. Water on land and of the ocean becomes water vapour in the atmosphere through the processes of evaporation and evapotransporation. When water vapour rises, it cools down and when it reaches a certain height, it condenses as cloud and then falls back to ground as precipitation.

Global warming (land and ocean) will affect the atmospheric moisture, precipitation and atmospheric circulation. Increases in temperature lead to increases in the moisture-holding capacity of the atmosphere and enhance the hydrological cycle, altering the characteristics of precipitation amount, frequency, intensity, duration, type, etc.

Analysis of long term data shows that, unlike the global temperature rise, the regional variation in precipitation trends is large. Some regions had a rising trend while some had a downward trend.

Water Cycle

The diagram shows the precipitation trends (1900 2005) at various regions. Precipitation curves with white background are having rising trends and those with yellow background falling trends. (Source: IPCC, 2007)

Yes. According to IPCC's Fourth Assessment Report, the observed decreases in snow and ice extent are consistent with the global warming. Mountain glaciers and snow cover on average have declined in both hemispheres. Since 1900, the extent of seasonal frozen ground in the Northern Hemisphere has decreased by about 7%, with a decrease in spring of up to 15%.

Average snow covered area in the Northern Hemisphere in March-April (Source : IPCC, 2007)

Satellite data since 1978 show that sea ice in the Arctic is shrinking in all seasons, especially in summer with a decreasing rate reaching 7.4% per decade.

Yes. There is strong evidence that global sea level gradually rose in the 20th century and is currently rising at an increased rate. Global average sea level has risen since 1961 at an average rate of 1.8 mm per year and since 1993 at 3.1 mm per year. The two major causes of global sea level rise are thermal expansion of the oceans (water expands as it warms) and the melting of ice and snow over ground (glaciers, ice caps, and polar ice sheets). Since 1993 thermal expansion of the oceans has contributed about 57% of the sum of the estimated individual contributions to the sea level rise, with decreases in glaciers and ice caps contributing about 28% and losses from the polar ice sheets contributing the remainder.

An extreme weather event (e.g. heavy rainfall, heat wave, cold spell, drought, etc.) is an infrequent event. Taking temperature as an example, the probability of occurrence of a temperature usually follows a normal distribution with a very low probability of occurrence (usually less than 5%) for extremely high or low temperature.

In a changing climate, a relatively small shift in the mean of the distribution can result in substantial changes in the frequency of extreme events. According to the Fourth Assessment Report of IPCC, over the last 50 years, there were widespread changes in extreme temperatures. In many places, hot days, hot nights and heat wave have become more frequent, while cold days, cold nights and frost have become rarer. Moreover, the frequency of heavy rain events has also increased over most land areas. Since the 1970s, more intense and longer droughts have been observed over wider areas, particularly in the tropics and subtropics. In Hong Kong, it is observed that cold episodes have become rarer while very hot days and heavy rain events are becoming more frequent over the last 120 years or so.

Taking temperature as an example, an increase in the mean can result in substantial changes in the frequency of extreme events.

It is not suitable to determine whether an individual extreme event is due to climate change alone because extreme weather events are usually caused by a combination of factors and a wide range of extreme events is a normal occurrence even in an unchanging climate. According to the Fourth Assessment Report of IPCC, observations over the past century suggested that the likelihood of some extreme events, such as heat waves and heavy precipitations, had increased due to climate change, and that the likelihood of others, such as cold spells and frost, had decreased.

Why snowstorms and extremely cold weather still occur in some regions under global warming?

The cold event in one place at one time (say, a week or a month) is just weather, and says nothing about climate. Global warming refers to a long term rising trend of globally averaged temperature attributed to human activity since the 20th century, in addition to natural climate variability. Snowstorms and extremely cold weather are extreme climate events against a background of rising temperatures. Such events are part of natural climate variability and are not precluded by global warming. However, global warming has reduced the frequency of occurrence of extremely cold weather over past few decades. The frequency of extremely cold events is expected to decrease further if the global temperature keeps rising in the future.

Recent satellite observations confirmed solar irradiance has an 11-year cycle related to sunspots. However, there is no increasing trend in solar irradiance in the last few decades, while global temperatures have increased significantly. Since the Industrial Revolution, additional man-made greenhouse gases have far more impact on the climate change than the variation of the Sun's irradiance. Therefore, solar activity is not the main cause of the climate warming in the 20th century.

Volcanic eruptions occasionally eject large amounts of dust and suspended particulates high into the atmosphere, temporarily shielding the Earth, reflecting sunlight back to space. This will decrease the solar energy received by the Earth's surface, causing short-term climate cooling.

The Earth's orbital variation brings itself closer or further away from the sun in predictable cycles, which could be related to the past ice-ages and very-long-term changes in the climate. Since the cycles have the periods of tens of thousands of years, they do not have much impact on the climate change observed over the past century.

Climate models cannot reproduce the warming observed in recent decades when only natural factors are considered. According to model simulation, we should have observed a decreasing trend in the global average temperature in the last few decades if only natural factors are considered, but we have observed a significant increasing trend in the global temperature. On the other hand, models can simulate the observed temperature changes in the 20th century when human factors, such as greenhouse gas emissions, are included. Therefore, it is very unlikely that the 20th century warming can be explained only by natural causes. Climate modeling results show that most of the global warming observed over the last 50 years is very likely due to human activities.

Modeling result of global temperature change by considering natural and anthropogenic factors. (relative to the corresponding average for 1901-1950) (Source: IPCC, 2007)

Human activities have increased the concentration of greenhouse gases in the atmosphere and enhanced the greenhouse effect, resulting in the accumulation of extra heat on Earth. The 'extra heat' has not only heated up the atmosphere and melted the ice/snow; it has also warmed up the oceans. The warming is not restricted to the sea surface. The sign of warming is observed down to 700m below the surface of the oceans. A direct consequence of ocean warming is the associated rise in the sea level. According to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), thermal expansion of sea water accounts for a rise of about 18 mm from 1993 to 2003 (i.e. about 1.6 mm per year).

The 2°C target was first put forward by European Union (EU) in 1996 based on the impact studies of the 2nd Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) published in 1995. During the 1939th Council Meeting held in Luxembourg in 1996, EU indicated that the global mean surface temperature increase should not exceed 2°C above pre-industrial levels to avoid the risk of severe climate change impacts on human and ecological systems. The 2°C increase above pre-industrial levels corresponds to a 1.4°C increase above 1990-2000 levels. The estimated temperature rise in 1990-2000 relative to the pre-industrial levels (1750s or 1850s) is about 0.6°C.

This threshold was later adopted by some other countries and widely cited by many researchers, green groups and organizations as the target ceiling of global warming.

Tropical cyclone is one of the most destructive weather systems on Earth. The possible change in tropical cyclone activity in a changing climate is a matter of great concern to the public and decision makers.

According to a study conducted by an expert team of the World Meteorological Organization (Knuston et al., 2010), it remains uncertain whether the changes in tropical cyclone activities based on the records of different basins in the last century or so, have exceeded the natural variability. This is because the trend detection is complicated by the large fluctuations in the frequency and intensity of tropical cyclones and the limitations in the availability and quality of global historical records. Looking into the future, theory and climate model simulations suggested that, if 21st century warming occurs as projected, the global frequency of tropical cyclones is expected to either decrease or remain unchanged. There will be some increase in the mean maximum wind speed of the tropical cyclones, although increases may not occur in all regions. The rainfall rates associated with tropical cyclones are likely to increase too.

In western North Pacific and the South China Sea (0-45°N, 100-180°E), analysis of the observational data shows that the annual number of tropical cyclones decreases from about 35 in the 1960s to about 27 after 2000. Locally, the annual number of tropical cyclones landing within 300 km of Hong Kong has decreased from about 3 in the 1960s to about 2.5 in the 2000s, but the trend is not statistically significant (Ginn et al., 2010).

Annual number of tropical cyclone landing within 300 km of Hong Kong (1961-2009)

The thermohaline circulation is a large-scale density-driven circulation in the ocean, caused by differences in temperature (thermo) and salinity (haline). It is also driven by mechanical forces such as winds and tides. In the North Atlantic the thermohaline circulation consists of warm surface water flowing northward and cold deep water flowing southward (see the figure), resulting in a net poleward transport of heat, thereby moderating the tropics and warming the high latitudes of Europe.

There are concerns that greater rainfall and melting of land ice and snow associated with climate change may change the salinity of the oceans and slow down or even halt the thermohaline circulation. Up to the end of 20th century, parts of the thermohaline circulation exhibit considerable inter-decadal variability, but data do not support a coherent trend. According to the Fourth Assessment Report of IPCC, it is very likely that the Atlantic thermohaline circulation will slow down over the course of the 21st century, but very unlikely that it will undergo a large abrupt transition.

The ability of different greenhouse gases in trapping heat differs due to their different physical properties, lifetimes and their concentrations in the atmosphere. For a given greenhouse gas, its warming effect (greenhouse effect) over a time period (say 100 years) relative to that of carbon dioxide (CO2) is represented by the Global Warming Potential (GWP). For example, the GWP (over a 100-year time frame) for methane is about 25 (25 times of that for CO2). This means, in terms of contribution to global warming, the emission of 1 tonne of methane is equivalent to that of 25 tonnes of carbon dioxide.

Based on the above concept, carbon dioxide equivalent (CO2-eq) is commonly used in carbon audit to gauge the combined greenhouse effect from a mixture of greenhouse gas emissions. CO2-eq refers to the amount of CO2, by weight, emitted into the atmosphere that would produce the same global warming potential as that of a given weight of other greenhouse gases being emitted. It is obtained by multiplying the emission of a greenhouse gas by its GWP for the given time horizon. As such, emission of 1 tonne of methane (GWP = 25) is equal to 25 tonnes of CO2-eq.

Although an individual extreme precipitation event cannot be solely attributed to climate change, as pointed out in scientific studies, climate change will likely affect the frequency of occurrence of such events in the long term. It is because the tropospheric warming due to increased anthropogenic (human induced) greenhouse gases can lead to an increase in the water-holding capacity of the atmosphere. The warming may also enhance the hydrological cycle and atmospheric instability. A less stable atmosphere with more water vapour in the air will provide a more favourable condition for intense precipitation events.

Moreover, some studies suggest that urbanization effect may also partly contribute to heavier rain in urban areas. This may be attributed to the urban heat island effect that enhances the convective activities, the increased roughness over a city that slows down the storm movement and the increase in the concentration of suspended particulates from urban activities that helps the formation and development of rain-bearing clouds.

Locally in Hong Kong, a study on the past occurrences of extreme rainfall indicates that heavy rain events in Hong Kong have become more frequent in the last 120 years or so.

Carbon dioxide emitted by volcanoes to the atmosphere is one of the natural factors contributing to variations in the ancient climate. However, various studies have shown that, in the last century, the annual amount of carbon dioxide released by human activities far exceeded that released by terrestrial and submarine volcanoes. The estimated amount of anthropogenic (i.e. human-induced) carbon dioxide emission in 2010 is about 35 gigatons, which is more than 100 times the estimated global volcanic carbon dioxide emission (about 0.26 gigaton per year).

Medieval warm period (MWP) generally refers to the warm period roughly around 900-1300 AD in some regions of the Northern Hemisphere (e.g. North Atlantic, Southern Greenland, the Eurasian Arctic, and parts of North America). Due to the scarcity of data prior to 1600, the precise duration and areal extent of the medieval warmth and whether it was a global phenomenon are still areas of active research. As pointed out by the United Nations Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) and some recent studies, there is no sufficient evidence to support that MWP was as warm as the 20th century as a whole. According to IPCC AR4, the warming in the late 20th century is widespread over the globe and the average Northern Hemisphere temperature in the late 20th century is likely the highest in the past 1300 years.

The causes of warming during MWP and the late 20th century are also different. The MWP was mainly due to natural factors, such as solar and volcanic activity. However, as indicated in IPCC AR4, most of the warming since the middle of the 20th century very likely results from the human-induced increase in atmospheric greenhouse gas concentration.

How do scientists reconstruct the temperature tens of thousands of years ago?

Snowfall in the polar regions accumulates from year to year. As snow at the surface gets buried with time, it is compressed to form solid ice. Lower sections of the ice sheet are therefore older than the upper ones. Ice cores drilled from the Greenland and Antarctica ice sheets containing proxy data of air temperature can then be used to estimate temperature in the past.

Natural oxygen (O) comes in two major varieties (or isotopes in chemical terminology): O18 and O16. O16 contains 8 protons and 8 neutrons and O18 contains 8 protons and 10 neutrons. O16 is lighter than O18 and is more common. Both isotopes can combine with two hydrogen atoms to form water molecules (H2O). The ratio of O18/O16 in the ice core is higher in a warmer climate as more energy is available to evaporate the heavy water molecules which contain O18 from the ocean and to transport them to the polar regions. In a cooler climate, fewer heavy water molecules can be evaporated into the atmosphere and even fewer of them can reach the polar regions before they condense out elsewhere. Hence, by determining the ratio of O18/O16 in different sections of the ice core, scientists can estimate the temperature in the past.

A variety of methods are used to date an ice core. The most direct way is to count the annual cycle of the O18/O16 ratio (the ratio is higher in summer and lower in winter). Another useful technique is to identify events (e.g. volcanic eruptions) which are seen in other types of climate records, such as tree ring and sedimentary records. These events can be used to synchronize the age scales and a time series of temperature over the past tens of thousands of years could then be reconstructed.

In May 2014, scientists claimed that the glacier loss in the Amundsen Sea sector of West Antarctica had passed the point of no return (see blog Point of no return), implying that the glaciers would eventually melt away.

The glaciers of concern sit on bedrock lying below sea level and flow towards the ocean across the grounding line, at which point they become floating ice shelf over the sea water. Using satellite data over the past two decades, researchers found that the grounding lines of the glaciers were all retreating inland rapidly, probably caused by warmer ocean currents under the floating ice. With the glacier beds below sea level and sloping downward in the inland direction, the more the grounding line retreats, the more vulnerable the ice becomes to the erosive effect caused by the incursion of warm ocean currents. Normally, ice outflow to the ocean increases with ice thickness at the grounding line. Here, the ice thickness at the grounding line increases as the latter continues to retreat, implying more ice outflow to the ocean. The only way to stop the vicious cycle of retreating grounding line and melting glaciers is to have some high obstructions rising from the bedrock at some point, which is exactly the natural blockage currently lacking in the landscape of West Antarctica.